This invention relates to electronics, and more particularly, to compositions useful for producing conductor patterns adherent to substrates.
Conductor compositions which are applied to and fired on dielectric substrates (glass, glass-ceramic, and ceramic) usually comprise finely divided inorganic powders (e.g., metal particles and binder particles) and are commonly applied to substrates using so-called "thick film" techniques, as a dispersion of these inorganic powders in an inert liquid medium or vehicle. Upon firing or sintering of the printed film, the metallic component of the composition provides the functional (conductive) utility, while the inorganic binder (e.g., glass, Bi.sub.2 O.sub.3, etc.) bonds the metal particles to one another and to the substrate. Thick film techniques are contrasted with thin film techniques which involve deposition of particles by evaporation or sputtering. Thick film techniques are generally discussed in "Handbook of Materials and Processes for Electronics," C. A. Harper, Editor, McGraw-Hill, N.Y. 1970, Chapter 12.
The most commonly used conductor compositions employ noble metals, especially gold, silver, platinum, palladium, and their mixtures, alloys, and compounds, since their relatively inert characteristics permit firing in air. Attempts at using dispersions of less expensive non-noble metals have often been confined to specialized uses or have required the great practical inconvenience and expense of firing in non-oxidizing atmospheres (nitrogen, nitrogen/hydrogen, hydrogen, argon, etc.).
There is a distinct commercial need for less expensive conductor compositions which can be fired in air to produce adherent low resistivity conductors on dielectric substrates, including microcircuit patterns, end terminations for capacitors, etc.
Present commercial practice in the manufacture of gas discharge display devices is to fire nickel powders in a reducing or inert (non-oxidizing) atmosphere at high temperatures (e.g., above 900.degree. C.) on relatively expensive forsterite (2MgO.SiO.sub.2) substrates. Nickel is used due to its low tendency to sputter under glow discharge. The use of inexpensive, mass-produced, high quality glass substrates such as soda lime glass substrates would be desirable. However, the use of soda lime glass substrates usually limits the firing temperature of conductors thereon to no more than 625.degree. C., due to the deformation of soda lime glass at higher temperatures. Firing at 625.degree. C. is possible when the glass is supported. Without a support, firing at or below 600.degree. C. is better, to prevent deformation. At these low temperatures it is very difficult to obtain good metallic sintering of nickel; consequently, adherent low resistivity nickel conductors are difficult to make.
There is consequently the need for a nickel-based conductor which can be fired below 625.degree. C. in making gas discharge display devices on soda lime glass substrates. It is further desirable that the compositions be fireable in air, rather than in more expensive atmospheres (inert or reducing atmospheres).
My U.S. Pat. No. 3,943,168 issued Mar. 9, 1976, discloses conductor compositions based on nickel boride (Ni.sub.3 B) and certain nickel boride/silicides. That patent is hereby incorporated by reference herein. It has been found that while Ni.sub.3 B gave improved results over the art, further improvement was needed in oxidation resistance during firing. Some oxidation of boron of Ni.sub.3 B to B.sub.2 O.sub.3 begins as early in the firing cycle as 325.degree. C., and the B.sub.2 O.sub.3 can prevent complete burnout of vehicle components. Also, during firing at 600.degree. C., nickel can begin to oxidize.